U.S. Pat. No. 4,724,532 describes a seismic vibration generator of the type specified in the preamble and operating by applying pressure variations to the hydraulic circuit acting on the reaction mass, where the pressure variations comply with a law defined by a predetermined pilot signal P(t) that is proportional to the reference emission signal F(t) that the operator desires to apply to the ground surface. In general, the pilot signal P(t) is an alternating signal of constant amplitude whose frequency is continuously variable, e.g. in the range 10 Hz to 70 Hz.
However, the large masses put into motion in such seismic vibration generators (which masses may be as much as 15 tons to 20 tons), and the coupling with the ground and the forces applied thereto make it difficult to servo-control the emission signal F(t) to the pilot signal. As a result the emission signal does not have the same shape as the intended signal F(t), so the seismic signal s(t) detected by the vibration sensors cannot be properly interpreted on the basis of the reference emission signal F(t). It is therefore essential to apply a correction to the seismic signal s(t) to take account of the difference in shape between the emission signal and the reference signal, if reliable and reproducible results are to be obtained.
Examples of interfering phenomena that may spoil the proportionality between the emission signal F(t) and the pilot signal P(t) include the following: rocking of the seismic vibration generator assembly, due for example to various causes such as the vibrating plate being placed in non-symmetrical equilibrium on the ground or the axis of the structure being non-vertical; overall or local deformation of the structure caused by the large mechanical stresses exerted thereon by the hydraulic system; deformation of the vibrating plate; and rocking of the reaction mass on its longitudinal axis, with such rocking being due to mechanical play between the vibrating mass and its drive piston having the effect of making the assembly constituted by the reaction mass and the vibrating plate different from a system having minimum phase shift relative to the modulation-controlling pilot signal. All this makes the generated vibration difficult to correct. In particular, the generated vibration includes a harmonic content which is difficult to correct, and interfering frequencies which interfere with servo-control and which are difficult to eliminate.
Thus, the technical problem to be solved by the present invention is to provide a correction method and correction apparatus making it easy to apply very accurate correction to the seismic signal s(t), to take account once and for all of all of the causes of difference between the emission signal F(t) and the reference emission signal F(t).